1. Field of the Invention
The present invention relates generally to the field of Local Oxidation of Silicon Isolation (LOCOS) forming technology, and more particularly, to a method of fabrication of modified poly-buffered LOCOS which can eliminate positive charge trapping at the beak of the field oxide.
2. Description of the Prior Art
LOCOS technology is currently employed most in the fabrication of Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and Bipolar Circuits to form the field oxide which acts as an isolation layer between active areas. A number of patents have addressed the LOCOS forming technology. U.S. Pat. No. 3,970,486 to E. Kooi, the entire disclosure of which is herein incorporated by reference, describes the processing steps as follows: A P-type substrate is provided, after which a layer of SiO.sub.2 and a layer of silicon nitride are deposited. The pattern of said silicon oxide and silicon nitride is formed by conventional lithography technology. The wafer is thermally oxidized to make the field oxide--the silicon nitride layer here acts as an oxidation mask to avoid the undesired oxidation of the active area. The P-type doping is usually implanted before or after field oxide forming. B.sup.+2 ions are implanted to build a P-type area in order to avoid the unwanted appearance of an inversion layer beneath field oxidation so it is helpful for insulation. The dose implanted before depositing of the oxidation layer is about 1E13 atoms/cm.sup.2 to 5E13 atoms/cm.sup.2 and the energy is of between about 110 to 130 KEV. The dose implanted after depositing of the oxidation layer is of between about 1E12 atoms/cm.sup.2 to 5E12 atoms/cm.sup.2 and the energy is of between about 170 to 190 KEV.
However, a so called "bird's beak" usually happens during LOCOS forming. Both the thinner silicon oxide layer and the thicker silicon nitride layer cause a longer bird's beak, and deteriorate the Crystal Defect of the silicon substrate. The lack of thickness of the bird's beak decreases the isolation ability of LOCOS as well. On the other hand, the active area is insufficient if the bird's beak is too thick. The bird's beak increases in thickness with the thickness of field oxide which increases during conventional processing. The thickness of the field oxide layer is usually increased in order to get enough threshold voltage and avoid leakage between devices, but a bird's beak of excessive length occurs at the same time. It affects the characteristic of the device and limits the ability to obtain high packing density.
U.S. Pat. No. 4,508,757 to Karen A. Fabricus, the entire disclosure of which is herein incorporated by reference, describes a method of manufacturing a minimum bird's beak recessed oxide isolation structure and approach called Poly-Buffered LOCOS process. The process steps are as follows: a silicon dioxide layer, a poly-buffered layer and a silicon nitride layer are formed in sequence. These layers are patterned by conventional lithography technology. The wafer is then subjected to an oxidation step employing said silicon nitride as an oxidation mask. The silicon nitride itself oxidizes very slowly, but is not penetrated by the oxygen. As a result, it protects the underlying silicon from oxidizing, while allowing a thermal SiO.sub.2 layer to grow on regions of exposed silicon. Then the nitride and poly-buffered layers are stripped and the other processes for formation of the device are continued. The poly-buffered layer of the '757 patent decreases the length of the bird's beak by providing a thinner silicon dioxide and thicker silicon nitride layer. Compared to the conventional length of the bird's beak (0.5 micrometer), the length of the bird's beak in devices employing the poly-buffered layer is about 0.1 micrometer. Therefore, it provides an approach to obtain high packing density devices. However, the process still has some undesired aspects because the poly-buffered layer tends to form SiO.sub.2 on its surface. For example, there is a significant level of positive charge traps at the PBLOCOS edge which cause early failure by charge breakdown.